An electrical double layer capacitor consists of a block containing at least two laminated units each comprising a pair of electrodes separated by a separator and positioned between a pair of collecting plates with an insulating ring being provided around the periphery of the electrodes, the block being contained between positive and negative cans. Alternatively, the capacitor may consists of one unit.
After the block is enclosed in the cans, voltage is applied to the capacitor to negatively charge the electrode on the negative can side and positively charge one on the positive can side. The thus charged capacitor can discharge for at least one month so that it is used as a power source and, after discharge, it is recharged and used as the power source again and again. Therefore, the capacitor finds application as a simple rechargeable power source of, for example, a watch.
Generally, the unit of the capacitor is produced by bonding each collecting plate to an upper insulating ring on the negative can side or to a lower insulating ring on the positive can side to form two so-called grommets, filling the cavity of each grommet with an electrode, and bonding the grommets by inserting the separator between them.
The electrode is formed in the grommet by applying a mixture of active carbon powder and an electrolytic solution on an inner wall of the collecting plate, or by compression molding the mixture in such a shape and size that it fits the shape and size of the cavity of the grommet and positioning the molded mixture in the cavity (cf. U.S. Pat. No. 3,536,963).
According to these methods, the electrode produced piece by piece by using a small amount of the electrode material results in poor productivity of the units and, in turn, the capacitor. Since the amount of active carbon powder is small, its filling density tends to fluctuate. Further, since the conventionally used active carbon powder has a comparatively small specific surface area of 1,000 to 1,500 m.sup.2 /g, a reaction area is insufficient so that it is difficult to increase and stabilize capacity of the capacitor.
Since the electrode produced by compression molding comprises active carbon powder and the electrolytic solution, its mechanical strength is rather poor. Therefore, it cracks or chips off during handling. This requires careful handling and results in fluctuation of capacity. This is one of the causes lowering the productivity of the capacitor.
It has been proposed to use active carbon fiber fabric as an electrode (cf. Japanese Patent Kokai Publication (unexamined) No. 99714/1980). Since the carbon fiber fabric has large specific surface areas of 2,000 to 2,500 m.sup.2 /g, it is expected that capacity per unit weight of the electrode is increased. Further, the carbon fiber fabric is excellent in handling and processability and has good mechanical strength. The active carbon fiber fabric has some drawbacks such that, since it is produced by carbonizing and activating fabric, its void ratio is so large that it is difficult to increase the filled amount of active carbon in the electrode. Further, contact resistance between the active carbon fiber and the collecting plate is so large that the capacity of the capacitor is hardly increased.